1. Field of the Invention (Technical Field)
The present invention relates to prevention or mitigation of spent nuclear fuel shipping cask weeping.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The phenomenon termed “weeping” is characterized by the occurrence of non-fixed, removable radioactive contamination on the surface of a radioactive material package, at a level exceeding regulatory limits and after prior demonstration that such removable contamination was within allowable limits. This is a persistent problem in the transport of spent fuel from commercial reactors. This problem can be traced primarily to two radioisotopes, 137Cs and 60Co, common in spent fuel storage pools.
The following lists prior art methods for the prevention or mitigation of cask surface contamination, with variable or unsubstantiated levels of success:                Spent-fuel pool cleanliness (contaminant level); pool chemistry (including lower pool pH)        Minimize cask immersion time (in spent-fuel pool)        Operational/administrative procedures                    Standardization of methods for contamination measurement and instrumentation.                        Minimize time interval between removal from the pool and start of decontamination.        Decontamination/cleaning detergents and agents (including chemical foams)                    Blocking agents            Low pH cleaning solutions            Pressurized water decontamination                        Cask surface finish                    Electro-polish cask surface            Stainless steel cask (or other material)                        Protective coatings/barrier on cask surface                    “Skirt” (mechanical barrier) around cask            Paint cask surface            Strippable paints            Cover cask during transport                        Cask design for ease of decontamination (surface finish and material; minimize protuberances)        Minimize stress to cask body and surface/air temperature gradients        
137Cs and 60Co are common radionuclides observed as non-fixed surface contamination when weeping incidents occur. A number of attempts have been made to provide a mechanistic explanation for the seemingly capricious occurrence of cask weeping incidents. Work has been performed at, or sponsored by, Sandia National Laboratories to determine the mechanism(s) of non-fixed surface contamination on package surfaces. This work led to the conclusions that the contamination is 1) an adsorption/desorption phenomenon; or is 2) a physical-chemical process in which radionuclides are incorporated (physically trapped and/or ionically bonded) onto the package surface. The reevaluation of the literature and the Sandia work by the present inventors suggested that while the phenomenological mechanisms previously conjectured were reasonable, a more likely mechanism for 137Cs weeping is ion exchange involving clay particles affixed to the exterior surfaces of shipping casks.
D. Jawarani, et al., “Critical discussion of relevant physical issues surrounding the weeping of nuclear-waste casks”, J. of Nuclear Materials, v. 206, p. 57-67 (1993), focused on the role that may be played by the passivating oxide layer on a stainless steel canister, in comparison with Co and Cs retention on painted surfaces. A number of important observations were made:                1. Metal oxides retain radionuclides by a “physical adsorption” mechanism while ion exchange plays a role in the case of painted surfaces. Once the radionuclide is on the surface sorption into the oxide surface coating occurs by diffusion.        2. On painted surfaces the titanium dioxide pigment may play a role in retaining Cs.        3. There is incubation time for the nuclides to reappear on the cask surface.        4. A potential role for road grime is noted but no mechanistic function is assigned to its presence other than to suggest that it initially scavenges radionuclides that are later somehow “locked” into the metal oxide surface coating.        5. Weeping is found to be strongly associated with surface oxide coatings that have developed to the extent that well-defined grain boundaries are present. Blistering, and presumably spalling, of these oxide layers (due to accumulated stresses) may account for weeping incidents. In fact, the oxide breakdown-repassivation cycle may occur numerous times.        6. The possibility of inhibiting the uptake of radionuclides by blocking the sorption sites using Ba2+ was postulated and then verified in a preliminary manner.Although quite general in application, the Jawarani model is somewhat vague regarding the kinds of sites that may be occupied by the various radionuclides in the oxide layers. It is unlikely that surface metal-hydroxyl sites are occupied by cobalt, and particularly not by cesium, since neither element is strongly sorbed by iron oxide. One tentative explanation offered is that radionuclides may be fixed at defects in the lattices where a free electron reduces the ion to a neutral metal atom. After that, it is hypothesized, the neutral atom diffuses with relative ease deeper into the metal oxide layer. In the case of silver, and to a lesser extent cobalt, this is plausible. Cesium, however, is so readily oxidized that this mechanism seems unlikely.        
Jawarani et al. also make reference to the importance of various environmental factors:                1. For painted canisters, exposure to humid conditions, or rain, increases the likelihood of weeping.        2. Weathering and exposure to abrasion, dirt, sun, and air all increase the chances of a weeping incident.        3. Low temperatures and low differential temperatures are associated with increased weeping but there is no association between weeping and precipitation or humidity.        
Weeping incidents usually are associated with releases of 60Co and 137Cs, though on occasion other radionuclides may be involved. Generally these two radioisotopes behave differently and may be influenced by different environmental factors. Studies to date have focused on the roles played by the surface coatings, paint and metal oxide passivating coatings, but have ignored the impact of road grime. As will become apparent, this component can significantly influence the retention and later release of 137Cs. In the course of the following discussion it should be kept in mind that the various mechanisms discussed in the literature—and the mechanism emphasized herein—are not mutually exclusive. Thus, depending on local history and environment all may operate in concert and it may not be possible to isolate a single cause for a particular weeping incident.
The present invention is of methods and kits for diminishing the incidents of radioactive 137CS contamination arising from the weeping phenomenon. A combination of pre-treatment (prior to placing a shipping cask into a spent fuel storage pool) and post-treatment (after it is removed) decreases residual 137Cs on the cask by a factor of 100 while also achieving smaller but still significant decreases in 60Co retention. Both pre- and post-treatments involve soaking the shipping cask surface in a solution of monovalent ions such as Cs+ (non-radioactive), K+, and NH4+. Pre-treatment works by blocking most of the sorption sites where radionuclide retention may occur. Post-treatment establishes conditions favorable to the displacement of sorbed radionuclides and thus facilitates removal of those radioisotopes that do become affixed to the cask surface as the spent fuel is being loaded. The invention is an improvement over the currently employed washing technology (often a rinse with soapy water) in that it targets specific chemical mechanisms (sorption, surface complexation, and ion exchange) responsible for retaining the chief radionuclides of concern during weeping incidents.